1. Field of the Invention
The present invention relates to a static eliminator and a static eliminating method for eliminating charges from an insulating sheet. Furthermore, the present invention relates to a method for producing an insulating sheet using said static eliminator or said static eliminating method, and also to an insulating sheet.
2. Description of the Related Art
The charges of an insulating sheet such as a plastic film can prevent the sheet from being processed as desired. As a result, it can happen that the quality of the processed sheet does not come up to the expected level. For example, in the case where a sheet having locally strong charges and discharge marks called static marks caused by electrostatic discharge is printed or coated with a coating material, the processed sheet has irregularity of the ink or coating material. In a process for producing a metallized film to be used, for example, in a capacitor or for packaging, the processed sheet can have static marks after completion of film processing such as vacuum evaporation or sputtering. The strong charges such as static marks cause the film to adhere to another member due to electrostatic force, hence causing such various problems as miscarriage, positioning failure and disarrangement of cut sheets.
The conventional static eliminators used to obviate such problems include the following: a self-discharge type static eliminator in which a grounded conductor shaped like a brush is brought close to the insulating sheet, to cause corona discharge at the tip of the brush for eliminating charges, and an AC or DC voltage application type static eliminator in which a power-frequency high voltage or DC high voltage is applied to a needle electrode to cause corona discharge for eliminating charges.
A conventional static eliminating method using corona discharge is described below. FIG. 1 is a drawing showing the principle of a conventional static eliminating method for an insulating sheet. In FIG. 1, a static eliminator 1 causes corona discharge by means of an ion generating electrode 1b connected to an AC power supply 1a and an earth electrode 1c, for generating positive ions 301 and negative ions 302 near the ion generating electrode 1b. Of the positive and negative ions, the positive ions 301 are attracted by an insulating sheet S due to the Coulomb force 700 acting between the positive ions 301 and the negative charges 102 of the sheet, to be balanced by the negative charges 102. As a result, the negative charges 102 of the insulating sheet S are eliminated.
However, actually, it is not rare that the charges of the sheet S are not eliminated according to the principle. The surface resistivities and volume resistivities of insulating sheets such as polyethylene terephthalate films, polypropylene films and aramid films used as photographic films, capacitor films and magnetic tape films are high. Therefore, the charges once generated in the sheet S can little migrate in the in-plane direction or in the thickness direction of the sheet. For this reason, if the potential of the sheet S rises with a large amount of negative charges accumulated, discharge can be caused between the sheet S and a grounded component used for carrying the sheet S or the like existing near the sheet S. In a sheet with a high surface resistivity and a high volume resistivity, since the migration of charges due to discharge is confined within local sites, it can happen that when discharge occurs, the local negative charges are excessively taken away to form sites having positive charges.
The discharge marks that are the marks of this discharge are static marks. If static marks are formed, there occurs a situation where positive charges 101 and negative charges 102 exist together in the sheet S. As shown in FIG. 2, if charges of positive polarity (positive charges 101) and charges of negative polarity (negative charges 102) are alternately formed at a small pitch, that is, if two kinds of charges with relatively high charge densities but opposite to each other in polarity exist close to each other, there occurs a phenomenon that the lines of electric force 500 attributable to the charges of the sheet S are closed between the respectively adjacent charged sites opposite to each other in polarity. Therefore, there occurs a situation where the Coulomb force 700 little acts on the ions near the static eliminator located a little away from the sheet S. As a result, ions are little attracted by the sheet S, and the charges 101 and 102 in the sheet S are little eliminated.
As shown in FIG. 3, there can be a case where positive charges 101, 201 and negative charges 102, 202 exist in both the surfaces of the sheet S. For example, in the case where a large amount of negative charges 102 exist in the first surface 100 of the sheet S, it can happen that discharge occurs between the sheet S and a grounded component (for example, a carrier roll) located close to the second surface 200 of the sheet. In this case, the negative charges 102 in the first surface 100 of the sheet remain also after discharge as they are, and the discharge causes sites having positive charges 201 to be formed in the second surface 200 of the sheet S. If such discharge occurs on both the first surface 100 and the second surface 200 of the sheet S, there occurs a situation where positively charged sites and negatively charged sites exist together in both the first surface 100 and the second surface 200 of the sheet S as shown in FIG. 3. Also in this case, the lines of electric force 500 attributable to the charges of the sheet S are closed between the negative charges 102 in the first surface 100 and the positive charges 201 in the second surface 200. So, Coulomb force does not act on the ions existing near the static eliminator either, and necessary ions cannot be attracted.
That is, in the case of a sheet having a fine charge pattern, i.e., a sheet where positively charged sites and negatively charged sites are alternately formed at a small pitch in one surface or where they exist together in both the surfaces, the lines of electric force 500 are closed near the sheet S. As a result, the Coulomb force 700 acting on the ions 301 and 302 located a little apart from the sheet S (near the static eliminator) is small, and the ions cannot be attracted toward the sheet S.
Measured charge densities of sheets having positively charged sites and negatively charged sites existing together in both the surfaces are stated in “Transactions on Fundamentals and Materials A (in Japanese), Vol. 112, No. 8, pages 735-740, The Institute of Electrical Engineers of Japan, August 1992 (hereinafter called document DS1).” According to the measured charge densities stated in document DS1, the charge densities in the first surface of a film as an insulating sheet are about −23 μC/m2, and the charge densities in the second surface of the sheet are about +23 μC/m2. In document DS1, the charges of such a film are called “both-side bipolar charges.”
On the other hand, the inventors confirmed the local charge densities at sites of sheets having a fine charge pattern such as static marks according to the method described later. As a result, it was found that there exist local sites having charge densities of about several to about 500 μC/m2 in absolute value in the respective surfaces, and that there exist some local sites in which the sums of the local charge densities of both the surfaces at the same sites in the in-plane direction of the sheet (apparent charge densities) were about 1 to about 40 μC/m2 in absolute value. These values are very large compared with the average charge densities generated due to the frictional electrification in an ordinary sheet production process. The average charge densities are said to be usually in a range from about 0.1 to about 1 μC/m2.
Especially it was found that in a fine charge pattern such as static marks, there were sites where the charge densities of the respective surfaces (for example, the charge density on the first surface 100 of a sheet was +500 μC/m2, while the charge density on the second surface 200 at the same position was −480 μC/m2) were far larger than the apparent charge densities (+20 μC/m2 in the above example) (usually about 1 to about 40 μC/m2 in absolute value). In the invention, the distribution of the quantities of charges in a sheet is mainly evaluated using the distribution of local charge densities. Unless otherwise stated, a charge density means the value of a local charge density of a sheet. As described above, in a sheet with a charge pattern such as static marks, the sums of charge densities of both the surfaces at the same site in the in-plane direction of the sheet (the apparent charge densities) are greatly different from the values of the charge densities of the respective surfaces at the same site.
In this specification, the sum of the (local) charge densities of both the surfaces at the same site in the in-plane direction of a sheet means the apparent charge density (the charge density identified without considering the distribution in the thickness direction) of the sheet at the site. This definition is important in the invention.
In the case where the apparent charge densities at the respective sites in the in-plane direction of a sheet are zero, the sheet appears to be non-charged, and in the case where they are not zero, the sheet appears to be charged. As described in document DS1, it has been known that an insulating sheet such as a film is bipolar-charged in both the surfaces. However, there is no report that has locally examined charge densities, and the description concerning static elimination relates to the apparent charges of a sheet. On the contrary, in discussing the statically eliminated state of an insulating sheet, the inventors have clarified that it is essentially important to examine both the apparent charge densities and the charge densities of the each surface.
For eliminating charges from an insulating sheet having such a charge pattern, usually a large quantity of the ions from a static eliminator are applied near to the sheet S without resorting to the Coulomb force acting due to the charges of the sheet.
As a technique for eliminating charges from an insulating sheet having such a charge pattern, a static eliminator as shown in FIG. 4 is known. The static eliminator 2 is disclosed in JP 2651476 C (hereinafter called document DS2). In FIG. 4, the static eliminator 2 consists of plural positive and negative ion-generating electrodes 2b connected with an AC power supply 2a and a planarly spread ion-attracting electrode 2d connected with an AC power supply 2c, and the positive and negative ion-generating electrodes 2b and the ion-attracting electrode 2d are installed to face each other through a traveling insulating sheet S. In the static eliminator 2, the positive and negative ion-generating electrodes 2b generate positive and negative ions, while high voltages opposite to the positive and negative ion-generating electrodes 2b in polarity are alternately applied to the ion-attracting electrode 2d, so that the positive and negative ions generated by the positive and negative ion-generating electrodes 2b can be attracted by the ion-attracting electrode 2d, to be forcibly irradiated to the sheet S.
As a result, positive and negative potentials are alternately induced in the sheet S, and the positive and negative ions from the positive and negative ion-generating electrodes 2b are forcibly attracted by the surface of the sheet S. So, it is said that even a sheet with a fine charge pattern can undergo static elimination. It is said that the statically eliminating action of the static eliminator 2 can be confirmed with a negative toner powder (black fine powder) used in a copier or the like to be electrostatically deposited on the sheet.
In this case, since the sheet is a thin insulator, the toner powder is deposited on the sites where the apparent charge densities are high. That is, a site where no toner powder is deposited means a site where the sheet is apparently non-charged (where the apparent charge density is almost zero).
However, the inventors confirmed that even if an insulating sheet is apparently non-charged by such static elimination, the sheet reveals its original charge pattern when it is processed to have a metalized film or to be coated. That is, it was found that the static eliminator 2 of document DS2 could not provide a sufficient static elimination effect. These can be actually confirmed since such defects as the irregularities of ink or coating material, static marks formed after such film processing as vacuum evaporation or sputtering, and disarrangement of cut sheets due to sliding failure actually occur. This is an essential problem, since the static eliminator of document DS2 can eliminate only the apparent charges described before.
This problem is described below in reference to FIGS. 5 to 7. In FIG. 5 and FIG. 6, an ion-generating electrode 2b is merely described to simplify the figure. It is assumed that in the sheet undergoing static elimination, positive charges 101 and 201 and negative charges 102 and 202 exist together in the respective surfaces 100 and 200 as shown in FIG. 5. As shown in FIG. 5, when the voltage applied to the positive and negative ion-generating electrode 2b is positive while the voltage applied to the ion-attracting electrode 2d is negative, the positive ions 301 generated by the positive and negative ion-generating electrode 2b are attracted near to the sheet S along the lines of electric force 500 generated by the positive and negative ion-generating electrode 2b and the ion-attracting electrode 2d, and are deposited on the first surface 100 of the sheet S, to positively charge the sheet S.
In this case, if there sites negative charges 102 exist in the first surface 100 of the sheet S, the positive ions 301 attracted selectively more to the sites than to their surroundings, for eliminating the negative charges. The reason is that since the positive ions 301 are carried near to the sheet S and go into the space where the charges 101, 102, 201 and 202 form the lines of electric force 500 closed near the sheet S, Coulomb force 700 acts between the positive ions 301 and those charges.
As shown in FIG. 5, in the case where the positive and negative charges 101, 102, 201 and 202 exist together in the respective surfaces 100 and 200 of the sheet S, the positive ions 301 are attracted more at the sites where the apparent charge densities are negative. That is, in the case where the positive charges 101 do not exist in the first surface 100 of the sheet S at the same sites in the in-plane direction of the sheet or in the case where even if the positive charges 101 exist, their quantity is smaller than the quantity of the negative charges 102 in the second surface 200 in the in-plane direction of the sheet, the positive ions 301 are attracted not only at the sites where only the negative charges 102 exist in the first surface 100 of the sheet S but also at the sites where the negative charges 202 exist in the second surface 200 of the sheet S.
Then, as shown in FIG. 6, if the voltage applied to the positive and negative ion-generating electrode 2b is switched to be negative (the voltage applied to the ion-attracting electrode 2d is positive), the negative ions 302 generated by the positive and negative ion-generating electrode 2b are attracted near to the sheet S along the lines of electric force 500 generated between the positive and negative ion-generating electrode 2b and the ion-attracting electrode 2d, and are deposited on the first surface 100 of the sheet S, to negatively charge the sheet S.
In this case, if there are sites having positive charges 101 in the first surface 100 of the sheet S, the negative ions 302 are attracted selectively more to the sites than to their surroundings, for eliminating the positive charges. Also in this case, the negative ions 302 are attracted more at the sites where the apparent charge densities of sheet S are positive.
Therefore, in the case where the negative charges 102 do not exist in the first surface 100 at the same sites in the in-plane direction of the sheet or in the case where even if the negative charges 102 exist, their quantity is smaller than the quantity of the positive charges 201 existing in the second surface 200 in the in-plane direction of the sheet, the negative ions 302 are attracted not only at the sites where the positive charges 101 exist in the first surface 100 of the sheet S but also at the sites where the positive charges 201 exist in the second surface 200 of the sheet S.
Since plural positive and negative ion-generating electrode 2b are installed in the traveling direction of the sheet, these actions are alternated, and the first surface 100 (the top surface in FIGS. 5 and 6) of the sheet S is alternately irradiated with positive and negative ions 301 and 302, to be positively and negatively charged, and accordingly the ions which are opposite in polarity to the apparent charges are selectively attracted, and eliminated apparently.
Since the irradiation quantities of positive and negative ions 301 and 302 depend, for example, on the capabilities of individual positive and negative ion-generating electrodes 2b and the phase of applied voltage, the total irradiation quantities of the positive and negative ions at the respective sites of the sheet S are different, and macroscopic positive and negative charge irregularity occurs in the sheet S (see FIG. 18 of document DS2). The macroscopic charge irregularity is the apparent charge irregularity and its state can be confirmed using a toner powder as apparent charges.
This occurs since the positive (or negative) ions 301 (or 302) are forcibly applied to the sheet S along the lines of electric force 500 generated by the positive and negative ion-generating electrodes 2b and the ion-attracting electrode 2d. Since the voltage applied to the positive and negative ion-generating electrodes 2b changes alternately, the cyclic irregularity of positive and negative charges occurs in the sheet S. The cycles of the charge irregularity are decided, for example, by the cycles of the applied voltage and the traveling speed of the sheet. The charge irregularity appears in the first surface 100 only of the sheet S. The reason is that the first surface 100 only of the sheet S is irradiated with the positive and negative ions 301 and 302, and this state shows that the sheet is apparently charged.
To eliminate the macroscopic charge irregularity, the static eliminator 2 of document DS2 must include DC and AC static eliminating members 2e and 2f shown in FIG. 4. The macroscopic charge irregularity can be eliminated if such conditions as the applied voltage and installation positions of the DC and AC static eliminating members are optimized. If the sheet is wound without the DC and AC static eliminating members, the charges are so strong that discharge may occur on the sheet. Since the static eliminator 2 of document DS2 requires such DC and AC static eliminating members, the entire eliminator is large-sized and very costly, and it is difficult to add the eliminator to an existing sheet producing apparatus.
On the other hand, the charged state of the sheet treated to be free from the macroscopic charge irregularity by the DC and AC static eliminating members 2e and 2f is as shown in FIG. 7. FIG. 7 shows a case where such conditions as the voltage and arrangement of the DC and AC static eliminating members 2e and 2f are optimized and where the macroscopic positive and negative charge irregularity in the sheet is eliminated. As shown in FIG. 7, the charges in the sheet S are balanced in both the surfaces, and the sheet S is apparently non-charged. However, in the respective surfaces of the sheet S, almost equal quantities of positive and negative charges remain.
The reason why this occurs is that the positive and negative ion-generating electrodes 2b are disposed only on the side of the first surface 100 (top surface in FIG. 5) of the sheet S, and hence that at every moment during static elimination, the charges in the second surface 200 (bottom surface in FIG. 5) of the sheet S cannot be decreased. This phenomenon occurs also in the case where the DC and AC static eliminating members 2e and 2f are used. As a result, the charge densities in the first surface 100 of the sheet S can be eliminated only to such an extent that the charge densities balance the charge densities prevailing in the second surface 200 since before static elimination, i.e., to such an extent that the apparent charge densities become zero.
The inventors measured, according to the method described later, the charge densities remaining in the respective surfaces of the sheet static eliminated by the conventional static eliminator 2. The charge densities at the static mark sites of the second surface 200 were virtually the same as those prevailing before static elimination, i.e., tens of microcoulombs per square meter to about 500 μC/m2 in absolute value. The charge densities of the first surface 100 at the same sites (static mark sites) were almost equal to those of the second surface 200 in absolute value, though opposite in polarity, i.e., tens of microcoulombs per square meter to about 500 μC/m2 in absolute value though opposite in polarity.
In view of the effect of decreasing the charge densities in the respective surfaces, the static elimination is achieved only to such an extent that the apparent charge densities (several microcoulombs per square meter to 10 μC/m2 in absolute value) are made zero. So, it can be said that the static elimination effect is only up to less than 10% of the charge densities of the first surface 100. Rather, such a phenomenon was also confirmed that at a site where the charge density of the second surface 200 was larger than the charge density of the first surface 100 before static elimination in absolute value, the charge density of the first surface 100 increased to such a level that it became equal to the charge density of the second surface 200 after static elimination. It was found that the charges remaining in the first and second surfaces 100 and 200 were the causes of such defects as the irregularity of the coating material, static marks formed after film processing and sliding failure.
This problem is an essential problem peculiar to the static elimination performed only from one surface of a sheet, and even if such conditions as the voltage and arrangement of the DC and AC static eliminating members 2e and 2f are optimized, the problem cannot be solved. The DC and AC static eliminating members 2e and 2f are provided only for making the macroscopic charge irregularity appear to be zero.
For example, two static eliminators of document DS2 (static eliminators 2 of FIG. 4) can be installed in the sheet traveling direction, and the two sets, each consisting of the positive and negative ion-generating electrodes 2b and the ion-attracting electrode 2d, can be arranged at positions facing each other, with the sheet kept between the electrodes 2b and the electrode 2d, and with one set reversed to the other set in position, in order that the first surface 100 of the sheet is irradiated with ions, and subsequently that the second surface 200 of the sheet is irradiated with ions. Even in this case, there is no effect of decreasing the charges existing in the respective surfaces. The reason is that the static eliminator of document DS2 (static eliminator 2 shown in FIG. 4) is a static eliminator intended for “apparent static elimination” only for eliminating apparent charges as described before. Even if static elimination is carried out for the second surface 200 after the “apparent static elimination” has been completed by the static elimination carried out for the first surface 100, the operation is quite meaningless.
On the contrary, as shown in FIG. 8, known is a static eliminator, in which ion irradiation devices, each consisting of an ion-generating electrode and an ion-accelerating electrode disposed to face each other, are installed reversely to each other in position on the first surface 100 side and the second surface 200 side of an insulating sheet. This static eliminator is disclosed in JP 2002-313596 A (hereinafter called document DS3).
The conventional static eliminator 3 includes an ion-generating electrode 3b connected with an AC power supply 3a and installed above the first surface 100 of a traveling insulating sheet S and an ion-accelerating electrode 3d connected with an AC power supply 3c and installed below the second surface 200 of the traveling insulating sheet S. The ion-generating electrode 3b and the ion-accelerating electrode 3d are installed to face each other with the insulating sheet S kept between them.
The next ion-generating electrode 3f connected with an AC power supply 3e and installed beside the ion-accelerating electrode 3d below the second surface 200 of the sheet S and the next ion-accelerating electrode 3h connected with an AC power supply 3g and installed beside the ion-generating electrode 3b above the first surface 100 of the sheet S, face each other.
In this static eliminator, an AC high voltage is applied to the ion-generating electrode 3b, to generate ions, and an AC high voltage opposite in polarity to the voltage applied to the ion-generating electrode 3b is applied to the ion-accelerating electrode 3d. The ions generated by the ion-generating electrode 3b are accelerated and attracted by the ion-accelerating electrode 3d, and as a result, the first surface 100 of the sheet S is forcibly irradiated with the ions. Then, an AC high voltage opposite in polarity to that applied to the ion-generating electrode 3b is applied to the ion-generating electrode 3f to generates the ions, while a high voltage opposite in polarity to that applied to the ion-generating electrode 3f is applied to the ion-accelerating electrode 3h. The ions generated by the ion-generating electrode 3f are accelerated and attracted by the ion-accelerating electrode 3h, and as a result, the second surface 200 of the sheet S is forcibly irradiated with the ions. According to this technique, since both the surfaces of the insulating sheet are forcibly irradiated with ions, it is said that the sheet can undergo static elimination even if the sheet has a fine charge pattern.
In this static eliminator, high voltages opposite in polarity to those applied to the ion-generating electrodes 3b and 3f disposed to face the ion-accelerating electrodes 3d and 3h respectively are applied to the ion-accelerating electrodes 3d and 3h restively. However, as shown in document DS3 (FIGS. 4 and 5 show examples of the shape of the ion-accelerating electrodes and FIG. 9 shows the behavior of ions), since the ion-accelerating electrodes are not shaped to allow ion generation, they do not generate ions. This is the reason why the electrodes are called “ion-accelerating electrodes” in document DS3. In this constitution, the irradiation of the first surface 100 and the second surface 200 with ions is carried out alternately, not simultaneously.
According to the inventors' finding, since both the surfaces of the insulating sheet are irradiated with ions alternately, the static eliminator of document DS3 is basically equivalent to the case where two static eliminators of document DS2 described before (static eliminators 2 of FIG. 4) are disposed in the sheet traveling direction, to be reverse to each other in the static elimination side and the non-static elimination side. That is, even in the best mode, quantities of positive and negative ions necessary to make the apparent charge densities zero are merely supplied without greatly affecting the distributions of charge densities existing in the respective surfaces before start of static elimination. In other words, at sites where a fine charge pattern such as static marks exists, a charge pattern opposite in polarity to the static marks of the first surface is merely formed in the second surface for apparent static elimination. That is, even if the static eliminator of document DS3 is used, an effect of greatly decreasing the charges in the respective surfaces where fine charge patterns are formed cannot be obtained.
This is described below in more detail. With regard to the capability of the static eliminator of document DS3 (static eliminator 3 of FIG. 8) to eliminate the charges in the respective surfaces of the sheet S (locally strong charges such as static marks, especially the charges opposite each other in polarity in both the surfaces of the sheet), the following can be said.
It is considered that a case where static elimination is performed at a site of a sheet where a large quantity of positive charges 101 in the first surface 100 and a large quantity of negative charges 202 in the second surface 200 exist as shown in FIG. 9. If the first ion-generating electrode 3b close to the first surface 100 of the sheet S generates the negative ions 302 to be sufficiently irradiated to the first surface 100 of the sheet S, and subsequently the second ion-generating electrode 3f close to the second surface 200 generates the positive ions 301 to be sufficiently irradiated to the second surface 200 of the sheet S, then the charges in the respective surfaces of the sheet S can be eliminated.
However, actually in the sheet S having the respective surfaces strongly charged opposite to each other in polarity, in the case where the negative ions 302 are irradiated to the first surface 100 of the sheet S as shown in FIG. 9, the positive charges 101 of the first surface 100 are eliminated. As a result, as shown in FIG. 10, the quantity of the negative charges 202 in the second surface 200 is excessive compared with the quantity of the positive charges 101 in the first surface 100.
In the case where a site of the sheet at which the absolute value of negative charge density of the second surface 200 is slightly larger, for example, 1 μC/m2 larger than the absolute value of positive charge density of the first surface 100 is placed in the space between the first ion-generating electrode 3b and the ion-accelerating electrode 3d, the potential is calculated to be in a range from −10 to −100 kV. This value range refers to a value range in the case where the electrostatic capacity of the sheet S placed in the space between the first ion-generating electrode 3b and the ion-accelerating electrode 3d is in a range from 10 to 100 pF.
Because of the excessively existing negative charges, the Coulomb force 700 in the direction to shove away the negative ions 302 from the sheet S acts on the negative ions 302, and the negative ions 302 cannot sufficiently reach the sites of the sheet S where the positive charges 101 still exist. Also in the case where the second ion-generating electrode 3f generates the positive ions 301 to be irradiated to the second surface 200 of the sheet S, the same phenomenon occurs. As a result, the positive charges 101 of the first surface become excessive, and the positive ions 301 reaching the sheet S decrease.
Even if the respective surfaces of the sheet S are charged to have charge densities of tens of microcoulombs per square meter to about 500 μC/m2 in absolute value, the quantity of ions per square meter that can reach the sheet S is as small as less than about 1 μC/m2, and can little eliminate the charges of the respective surfaces of the sheet S so strongly charged as to have static marks. However, at each site where the apparent charge densities of the sheet are not zero, the charges can be eliminated to such an extent that the apparent charge densities can be made zero.
As a mode of the static eliminator of document DS3, the following constitution is described in FIG. 2 of document DS3. Ion irradiation devices, each consisting of the ion-generating electrode 3b and the ion-accelerating electrode 3d facing each other, are arranged on both the surface sides of the sheet S, with the electrodes disposed alternately in reverse positions, and on the downstream side, two ion-generating electrodes are arranged to face each other on both the surface sides of the sheet S, one on the first surface 100 side and the other on the second surface 200 side. The ion-generating electrodes disposed downstream to face each other are disposed to eliminate the residual charges (same as the charges of macroscopic charge irregularities of static eliminator 2 of FIG. 4.) However, for example, the dimensions and applied voltages of the ion-generating electrodes disposed downstream to face each other are not disclosed at all in document DS3.
Even if a voltage considered to be appropriate is applied to the ion-generating electrodes disposed to face each other, based on the inventors' finding, it is difficult to obtain a sufficient static elimination effect. For example, if the ion-generating electrode placed on the first surface 100 side of the sheet S generates positive ions to be irradiated to the first surface 100, and the ion-generating electrode placed on the second surface 200 side generates negative ions to be irradiated to the second surface 200, then a static elimination effect can be obtained at sites where the first surface 100 is charged negatively while the second surface 200 is charged positively. However, no static elimination effect can be obtained at the sites where the first surface 100 is charged positively while the second surface 200 is charged negatively.
Since positive charges and negative charges exist together in the respective surfaces of the sheet S in most cases, the charges at all the sites in the respective surfaces of the sheet S cannot be decreased. There are sites where charges can be eliminated and sites where charges cannot be eliminated. Rather, it can happen that in the case where the polarity of charges of the respective surfaces of the sheet S is the same as the polarity of the ions irradiated to the respective surfaces, charges are increased. In the case where the voltages applied to ion-generating electrodes are AC voltages with a low frequency, static elimination effect irregularity and ion irradiation irregularity appear in the traveling direction of the sheet S. On the other hand, in the case where the voltages applied to ion-generating electrodes are AC voltages with a high frequency, the static elimination effect irregularity in the traveling direction of the sheet S is small.
However, in the case where the voltages applied to ion-generating electrodes are AC voltages with a high frequency, as in the case of a static eliminator for a copier described later, since the positive and negative ions generated from ion-generating electrodes are mixed and re-combined with each other before they reach the sheet S, the quantity of ions reaching the sheet S is remarkably decreased. Therefore, the static elimination effect per se is small. So, even if, for example, the dimensions of respective parts and the applied voltage are adjusted based on the inventors' finding, it is difficult to eliminate the positive charges and negative charges existing together in both the surfaces without the irregularity due to the positions in the traveling direction of the sheet S, if one set of ion-generating electrodes, one on the first surface 100 side of the sheet S and the other on the second surface 200 side, are merely disposed.
On the other hand, as a constitution in which static eliminators are disposed to face each other with a charged material positioned between them, a transfer sheet-carrying sheet and a transfer sheet (paper) static eliminator 4 of a copier shown in FIG. 11 is known. The static eliminator 4 is disclosed in JP 03-87885 A (hereinafter called document DS4) or JP 02-13977 A (hereinafter called document DS5).
FIG. 11 is a drawing showing the copier shown in document DS4, as a whole. In FIG. 11, A indicates a section for forming a toner image onto a photosensitive drum; B indicates a section for supplying a transfer sheet 4a; C indicates a section for transferring a toner image onto the transfer sheet 4a on a transfer sheet-carrying sheet 4b wound around a transfer drum; and D indicates a section where the transfer sheet 4a having the toner image transferred from the transfer sheet-carrying sheet 4b is separated. The description of the details is not made here since it is not concerned with the present invention at all.
In the static eliminator 4 of FIG. 11, wire corotron electrodes positioned outside as corona dischargers 4c and 4d and wire corotron electrodes positioned inside as corona dischargers 4e and 4f are installed to face each other on both sides of the transfer sheet 4a as a charged material and the transfer sheet-carrying sheet 4b. The first purpose of the static eliminator 4 is to more easily separate the transfer sheet 4a from the transfer sheet-carrying sheet 4b, and the second purpose is to initialize the potential of the transfer sheet-carrying sheet 4b. 
To achieve the first purpose, an AC voltage (500 Hz, 9.6 kV) is applied to the corona dischargers 4c and 4d, and a DC voltage (−4 kV) is applied as pulses to the corona discharger 4e, while a voltage different by 180° phase from that of the corona dischargers 4c and 4d is applied to the corona discharger 4f. The reason why a DC voltage is applied to the corona discharger 4e is that instead of superimpose a DC voltage as a bias on the AC voltage applied to the corona discharger 4f in opposite, it is intended to use two independent corona dischargers 4f and 4e. 
With this constitution, the average potentials of the transfer sheet 4a and the transfer sheet-carrying sheet 4b can be decreased. Since the transfer sheet 4a is positively charged in the previous step, a negative voltage is used as the DC voltage to allow easier separation of the transfer sheet-carrying sheet 4b. To achieve the second purpose, an AC voltage only is applied to the corona dischargers 4d and 4f. With regard to the charges of the transfer sheet-carrying sheet 4b, it is not necessary to eliminate the charges of both the outer surface and the inner surface. If the charges of the outer surface balance the charges of the inner surface to reduce the apparent potential to almost zero, the purpose can be achieved.
As can be seen from the above description, the technique described in document DS4 is not intended to eliminate charges from a sheet having positively charged sites and negatively charged sites alternately formed at a small pitch in the same plane or a sheet having fine patterns with such sites existing together in both the surfaces. In the paper as a transfer sheet of a copier, such charge patterns are unlikely to be formed.
In the case where such a high frequency is used, the electric field between the top and bottom electrodes little has the capability of forcibly irradiating the sheet with ions. The positive and negative ions 301 and 302 generated by the corona dischargers 4d and 4f are mixed in the gap between the corona discharger 4d and the corona discharger 4f. The size of the gap is not clearly stated in document DS4, but according to other documents and the like relating to static eliminators of copiers, it is usually about 20 mm. According to document DS5, it is 22 mm.
Since an AC voltage with a high frequency of 500 Hz is applied in an electrode gap of about 20 mm as described above, a monopolar ion cloud cannot be formed. Since the frequency is high, the positive and negative ions 301 and 302 are mixed with each other, before they reach the first surface 100 and the second surface 200 of the sheet. For this reason, though the sheet is seldom forcibly charged positively or negatively, most of the positive and negative ions 301 and 302 are recombined with each other and vanish, and the quantity of the ions capable of contributing to static elimination becomes very small. That is, in the static eliminators shown in documents DS4 and DS5, though the corona discharger 4d and the corona discharger 4f are disposed to face each other with a sheet kept between them, a large quantity of ions can be little forcibly irradiated near to the sheet.
As a result, these static eliminators of copiers, like the static eliminator 1 shown in FIGS. 2 and 3, are very low in the capability of eliminating the charges of the respective surfaces of a sheet having positively charged sites and negatively charged sites alternately formed at a small pitch in the same plane or a sheet having such sites existing together on both the surfaces. The techniques can be applied in the case where the sheet traveling speed is as low as several to 10-odd m/min and can be applied to a transfer sheet or paper from which it is not required to eliminate the fine charge patterns in either of the surfaces. The static elimination techniques cannot be applied as techniques for eliminating charges from an insulating sheet such as a film that travels at a high speed of about 50 to about 500 m/min and from which it is necessary to eliminate fine charge patterns in both the surfaces.
Furthermore, in the static eliminators for copiers shown in documents DS4 and DS5, the width of the transfer sheet or paper undergoing static elimination is about 500 mm at the largest, and it is not necessary to consider, for example, the vibration, strength and sagging of electrodes. For this reason, a high voltage is applied to wire electrodes extending in the in-plane direction perpendicular to the traveling direction of the sheet, for causing discharge to generate ions. However, in the case where an insulating sheet such as a film undergoes static elimination, its width is about 1 m at the smallest, and there is even an insulating sheet with a width of about 7 m. When wire electrodes are used for such a wide sheet, the vibration of the electrode and the sagging of the electrode between both the ends cause discharge strength irregularity in the sheet width direction.
For example, in the case where it is intended to increase the ion irradiation dose for the sheet undergoing static elimination, for example, by further shortening the distance between the corona discharger 4d and the corona discharger 4f, or raising the voltage to be applied, or using a lower frequency, the vibration of the wires increases, and discharge is concentrated at the portion where the distance between the wires facing each other is shortest due to inaccurate parallelism or loosening of wires. As a result, a static elimination effect stable over the entire width of the material undergoing static elimination cannot be obtained. Furthermore, in the case where the voltage is raised, spark discharge occurs between the discharge electrodes (wire electrodes) of the corona dischargers 4d and 4f or between a discharge electrode and a shield electrode, not allowing a sufficient static elimination capability to be obtained.
In the static eliminators for copiers shown in documents DS4 and DS5, corona dischargers are disposed to face each other, but the principle of static elimination is quite different from the principle that a strong electric field in the direction normal to the insulating sheet is used to forcibly irradiate ions onto the sheet. Therefore, the static elimination irregularity in the traveling direction of the sheet is hard to occur, and no countermeasure against it is discussed at all. For example, in the static eliminator shown in document DS4 (the static eliminator 4 of FIG. 11), two sets of corona dischargers facing each other are installed one after another in the traveling direction of the material undergoing static elimination (transfer sheet or paper), but as described before, this constitution is intended to provide different functions of easier separation and potential initialization, and is not employed to give any effect, for example, against the static elimination effect irregularity in the traveling direction of the sheet.
In recent years, insulating sheets such as polyester films are used in many applications as magnetic recording materials, various photographic materials, insulating materials and various process materials, since they have excellent properties such as heat resistance, chemicals resistance and mechanical properties. For this reason, they are required to have surface properties suitable for respective applications, and they are covered with various materials. For example, the sheets are thinly coated on their surfaces with a magnetic paint, ink-like paint, lubricating paint, releasing paint, or hard coating material, to form a coating layer.
For the coating process for forming such a coating layer, it is proposed to install a static eliminator in any of various coaters such as roll coater or gravure coater, for eliminating the charges from an insulating sheet before start of coating, or to eliminate charges from the sheet and a coating solution simultaneously before the coating solution applied as a paint is dried after coating. These proposals are described in JP 08-334735 A (hereinafter called document DS6) and JP 10-259328 A (hereinafter called document DS7). As the quantity of charges of a sheet for obviating the occurrence of coating irregularities, document DS6 states it is preferred that the surface potentials of the sheet are in a range from 0 to 80 V, and document DS7 states it is preferred that the surface potentials of the sheet are in a range from 0 to 2 kV.
In these conventional techniques, the surface potential refers to a value measured while the sheet is carried in air. Hereinafter this surface potential is called an aerial potential. In the state where a sheet is carried in air, since the thickness of the sheet is sufficiently small compared with the distance between the sheet and a grounded component, the surface potential corresponding to the sum of charges is measured without discriminating the charges of the first surface of the sheet from the charges of the second surface. That is, in these conventional techniques, the aerial potential relates to apparent charges (the apparent charge densities). Therefore, in the conventional techniques, the charge densities of the respective surfaces of a sheet are not taken into account at all.
The visual field of a general electrostatic voltmeter used for measuring the aerial potential is usually a virtually circular area portion having a diameter of tens of millimeters to tens of centimeters, and the value of the measured potential is detected as an average value of potentials in the visual field. This matter is described in the catalogue (in Japanese) for Digital Low Potential Measuring Instrument KSD-0202 produced by Kasuga Electric Works Ltd (hereinafter called document DS8). In a dense charge pattern having positive and negative charges existing together peculiar to an insulating sheet, the positive and negative charges are averaged within the range of the visual field, and the aerial potential appears to be almost zero. With these as causes, even in a sheet having a low aerial potential according to the conventional techniques, it can happen that numerous positive and negative changes exist in the sheet actually, and in this case, coating irregularity occurs in the coating layer.
As described above, even if the above-mentioned sheet having positively and negatively charged sites alternately formed at a small pitch or having such sites existing together in both the surfaces has its charges controlled in reference to the aerial potential, the control is not sufficient. Much less, the coating irregularity can never be prevented.
The following describes why an apparently non-charged sheet having both the surfaces equally charged though opposite in polarity (in this case, the aerial potential is also zero) poses a problem and why coating irregularity occurs.
In a coating process, for example, when a die coater is used, the sheet travels, for example, with its second surface kept in contact with a backup roll. In this state, a coater roll is used to coat the first surface of the sheet. Since the sheet is kept in contact with the backup roll, stable traveling is assured to stabilize coating work, and a coating layer having uniform thickness can be formed. As the material of the backup roll, a metallic material is often used since the roll is required to be mechanically precise and to have durability such as wear resistance. Therefore, one surface of the sheet is kept in contact with the metallic surface of the backup roll, and the other surface is coated to have a coating film.
It is considered that a sheet having the first surface and the second surface charged equally though opposite in polarity (apparently non-charged sheet). The charges of the second surface in contact with the metallic surface induce an equal quantity of charges opposite in polarity in the surface of the metal that is a conductor. The induced charges opposite in polarity apparently cancel out the charges in the second surface. On the other hand, the charges in coating surface (the first surface) also induce charges opposite in polarity in the surface of the metal. However, since the surface of the metal is far in this case, the quantity of charges induced is smaller. Therefore, the induced charges opposite in polarity do not perfectly cancel out the charges of the first surface, and the charges actively exist in the coating surface (the first surface).
In this way, “the apparently non-charged” sheet have charges actively existing in the first surface above the backup roll during coating. Therefore, coating irregularity occurs. That is, even in an apparently non-charged sheet, as far as charges exist in the respective surfaces of the sheet, coating irregularity can occur. This phenomenon occurs also similarly in the carrier roll or drying roll used after coating.
As described above, even if the aerial potential of a sheet is kept low as in the prior art, and furthermore, even if apparent charges are used for control, the prior art cannot prevent coating irregularity.